The difference is that OEM comes without any packaging and directions like you would get in a retail package , also the warrenty is strickley limited to shipping and the standard 30 days for defects. You don't get any tech support or sutomer service other than the store that is selling the item. These items are usually sold to computer builders that put together thier machines for sale , Dell , HP , Sony and so on. You provide your own tech support. This kind of thing works out ok for basic stuff like hard drives and dvd roms and power supplies but for cpu's , ram , motherboards and video cards it is probably better to have the retail item.